One has to remember that he was pretty much hated when he first took the title, bragging beforehand that he would take Sonny Liston.
That might be courage born out of brash youth, but his personal belief that the way the draft worked was discriminatory, and that he was being used merely as entertainment for white America, and that he would not be accepted equally, these are things that gave him a dimension outside of boxing.
When he made his stand against the draft he was not fully aware of the issues but it just felt wrong to him, and he got loads of abuse and approbrium for it by the whole of establishment America.
This was before the civil rights movement had got going so he really took all the pressure by himself, Malcolm X was not there to support him, nor was Martin Luther King, and many other Black Americans who had been drafted disliked him.
There he was, a young man only , what? 24 ?, alone with his principles and the American power brokers against him, he refused any deals to keep out of the combat zone, totally uncompromising.
He got immense flak from political America who saw his refusal to enlist as a worrying role model for all the other cannon fodder they intended to send to Vietnam.
In the end he took back his title from those unworthies who had stolen it from him in courtrooms instead of the ring, and he was always good for a quote,
“I got nothing against no Viet Cong. No Vietnamese ever called me a nigger.”
Summed up the racism in the US of the 60’s, why would he want to fight for a country that viewed him as a second class citizen.
All this from the sort of person who was usually regarded as being maybe a great athlete but lacking in the intelligence department, this was the stereotype the public had of the heavyweight boxer especially black ones, Ali shattered all these assumptions.